Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Many modern vehicles are incorporating heads-up-display (HUD) devices to more safely display useful information to drivers. HUDs are typically either incorporated into the vehicle dash upon manufacture or added as a separate off-the-shelf dash-mounted module. These HUDs project an image vertically onto the angled windscreen such that a virtual representation of the image is seen by the driver in a lower region of the windscreen. HUDs display useful information to drivers without significantly distracting the driver from the forward road scene. Such useful information includes the current vehicle speed, GPS navigation signals, alerts and indicators.
The dominant technology currently used in HUDs is LED backlit Liquid Crystal Displays (LCDs). These backlit devices necessarily require significant power to sufficiently illuminate the image as a portion of the light is absorbed upon passing through the LCD. These devices also have limitations in terms of contrast and dynamic range. Alternatively the use of mirror matrices (for example, Texas Instruments DLP projectors) can be used in a HUD but they are also inefficient in optical power usage for a display with sparse information such as is common for an instrument display. These technologies do not provide naturally for a variable perceived distance of the virtual object which is of great value in a HUD environment to convey simple information about location of dangers or navigational features such as upcoming intersections.
More recently, phase only holography based HUD systems have been proposed. These systems utilize a Liquid Crystal On Silicon (LCOS) spatial light modulator to generate a holographic image diffraction pattern. Coherent light is projected onto the LCOS and a holographic image is generated at an image plane corresponding to the spatial Fourier transform of the diffraction pattern. More generally a holographic projection scheme is one which utilizes the coherent phase of light to create an optical field that later propagates to form an image. It is in this more general sense that the terms ‘holographic projection’ and ‘holographic image’ will be used in this specification. The holographic image is projected onto a vehicle windscreen such that the driver sees a virtual representation of the holographic image at some distance in front of the windscreen. Existing LCOS based holographic projectors rely on complicated, costly and bulky lensing systems (of long focal length) to both remove the zero-order diffraction component (which would significantly degrade contrast by providing a background optical power) and to create an intermediate image of sufficient lateral extent to provide an extended angular field of view. For a given focal length or projection distance, the lateral extent of the intermediate image is constrained by the pixel size due to the physics of diffraction and reducing pixel size can come at a cost of reduced performance. In particular, reduced pixel size can give rise to increased phase flicker, which strongly affects the contrast achievable and the reduction of order artifacts which can arise in holographic projection systems. By way of example, US Patent Application Publication 2013/0265622 to Christmas et al. entitled “2D/3D Holographic Display System” removes the zero-order diffraction component through a spatial filter at the image plane. FIG. 1 illustrates schematically the optical layout of the display system of Christmas et al.
There is a desire in the art for cheaper, more compact HUD projection systems having small throw distances.